Power generation utilizing natural energy such as sunlight or wind power is generally susceptible to environmental influence, and the amount of power generation fluctuates greatly. A system stabilizing device is used for the purpose of accommodating or absorbing this fluctuation.
In a micro grid having a network constructed by installing a power source, such as natural energy, near the electric power demand side, the installation of a system stabilizing device, which keeps demand for and supply of electric power in balance, is necessary for system stabilization.
An example of the micro grid (electric distribution or distribution system) equipped with a system stabilizing device will be described by reference to FIG. 3. FIG. 3 shows an example in which an existing superior power system (superior distribution system) 1 and a distribution system (micro grid) 10 are connected via a line impedance Ls and a circuit breaker 2.
A dispersed generation plant 11 and a load 12 are connected to the distribution system 10 which is the micro grid. The dispersed generation plant 11 is illustrated as a single generator in FIG. 3. Actually, however, it is composed of a plurality of dispersed facilities for power generation, which include natural energy type power generation equipment utilizing natural energy (e.g., photovoltaic power generation equipment or wind power generation equipment), and internal combustion engine type power generation equipment driven by an internal combustion engine (e.g., diesel power generation equipment). Also, the load 12 is actually a plurality of dispersed loads.
With the micro grid 10 shown in FIG. 3, the amount of power generation varies or fluctuates greatly according to weather conditions, wind speed, etc., because it has natural energy type power generation equipment.
In order to accommodate such fluctuations in the amount of power generation, therefore, a system stabilizing device 20 is used.
With the internal combustion engine type power generation equipment, output power is adjusted by governor control. However, governor control is slow in response. Thus, if electric power consumed by the load 12 suddenly changes, the internal combustion engine type power generation equipment cannot follow such a sudden change (sudden excess or deficiency) in electric power.
The system stabilizing device 20 is used for the purpose of following such a sudden change in electric power with good response, thereby assisting the internal combustion engine type power generation equipment to balance demand for and supply of electric power.
The system stabilizing device 20 is a power converter having a power storage function, and is provided in the distribution system 10 in a state connected in parallel with the dispersed generation plant 11 and the load 12.
The system stabilizing device 20 has a self-supporting control unit 21, an interconnected control unit 22, a change-over switch 23, a current control unit 24, a PMM (pulse width modulation) modulator 25, a power converter 26 capable of an inverting action and a converting or rectifying action, and a direct current charging unit 27 such as an electric double layer capacitor or a battery.
The power converter 26 acts responsive to a gate signal g fed from the PWM modulator 25. This power converter 26, when performing a converting action, converts an alternating current power obtained from the distribution system 10 into a direct current power, and charges this direct current power into the direct current charging unit 27. When performing an inverting action, the power converter 26 converts the direct current power charged in the direct current charging unit 27 into an alternating current power, and sends this alternating current power to the distribution system 10.
In the system stabilizing device 20, moreover, a system current Is, which flows from the power system 1 into the distribution system 10, is detected by a current detector 28, a system voltage Vs which is the voltage of the distribution system 10 is detected by a voltage detector 29, and an alternating current (AC) output current Iinv outputted from the power converter 26 is detected by a current detector 30.
Under normal conditions where no breakdown or the like occurs in the power system 1, the circuit breaker 2 is in a connected state, so that “a system-interconnected run”, an operation with the distribution system 10 being tied to the power system 1, is performed in the system stabilizing device 20. During the system-interconnected run, electric power is supplied to the load 12 by the power system 1, the dispersed generation plant 11, and the system stabilizing device 20.
During this system-interconnected run, a movable contact 23a of the change-over switch 23 is thrown to the A side as indicated by a dashed line in FIG. 3. As a result, the gate signal g obtained under control of the interconnected control unit 22 is fed to the power converter 26 to actuate the power converter 26.
During the above system-interconnected run, the system stabilizing device 20 acts to detect the system current Is flowing into the distribution system 10, determine a system power from the system current Is, and suppress a fluctuation in this system power. That is, the system stabilizing device 20 acts to detect a power flow at the point of interconnection between the distribution system (micro grid) 10 and the power system 1 and render fluctuations in the power flow gentle.
Under abnormal conditions where a breakdown occurs in the power system 1, on the other hand, the circuit breaker 2 is in a shut-off state, and the system stabilizing device 20 makes a “self-supporting run”, a run performed with the distribution system 10 being cut off from the power system 1. During the self-supporting run, electric power is supplied to the load 12 by the dispersed generation plant 11 and the system stabilizing device 20.
During this self-supporting run, the movable contact 23a of the change-over switch 23 is thrown to the B side as indicated by a solid line in FIG. 3. As a result, the gate signal g obtained under control of the self-supporting control unit 21 is fed to the power converter 26 to actuate the power converter 26.
During the above self-supporting run, the system stabilizing device 20 detects the system voltage Vs within the distribution system 10, and performs a compensating action so that the voltage amplitude and frequency of this system voltage Vs become stable. The system stabilizing device 20 detects an excess or deficiency in the power within the distribution system (micro grid) 10, and exercises input or output control over the power. That is, the system stabilizing device 20 exercises such control as to charge surplus power into the direct current charging unit 27 when the power within the micro grid 10 is greater than the load power, and to output the charged power when the load power is insufficient.
Details of the actions of the system stabilizing device 20 during the self-supporting run will be described by reference to FIG. 4.
During the self-supporting run, power is supplied from the dispersed generation plant 11 to the load 12. When the power load sharply increases at this time, the torque of the generator becomes insufficient for the load power, so that the number of revolutions decreases to lower the frequency of the voltage.
Governor control for maintaining the frequency of output voltage at a constant value is applied to the internal combustion engine type power generation equipment. However, governor control is slow in response, so that if the load sharply increases, a decrease in the frequency lasting for several seconds or so occurs. As noted here, a load change (sharp increase in load) causes a great change to the frequency (frequency decrease). According to this change, other power generation equipment within the micro grid 10 also increases in load, and governor control is performed for the other power generation equipment. It follows that governor control is exercised in a plurality of power generation equipment. If such a plurality of governor controls interfere with each other, oscillations, etc. occur in the system voltage, rendering power supply from the dispersed generation plant 11 to the load 12 unstable.
Under this situation, upon detection of a decrease in the frequency of the system voltage Vs, the system stabilizing device 20 outputs an effective or active power to assist governor control effected by the internal combustion engine type power generation equipment, keeping a decrease in frequency to a minimum.
If the load increases, a voltage drop in the system voltage Vs is aroused by an armature reaction L within the generator of the power generation equipment.
Under this situation, upon detection of a voltage drop in the system voltage Vs, the system stabilizing device 20 acts as a capacitor load, that is, outputs a reactive power, to cancel out the voltage drop in an armature inductance LG, thereby suppressing the voltage drop in the system voltage Vs.
By performing the above-mentioned two types of actions, the system stabilizing device 20 suppresses fluctuations in the frequency and amplitude (voltage value) of the system voltage Vs to improve power quality.
By further reference to FIG. 4, explanations will be offered for the configurations and actions of respective functional blocks which act during the self-supporting run among the respective functional blocks of the system stabilizing device 20.
A zero-crossing detecting unit 40 takes in the system voltage Vs detected by the voltage detector 29, and outputs a zero-crossing signal Z showing the interval between the zero-crossings of its sinusoidal waveform. A frequency converting unit 41 outputs a frequency signal ωs showing the frequency of the system voltage Vs based on the zero-crossing signal Z.
A fluctuation detecting unit 42 determines the fluctuation component of the frequency signal ωs, and this fluctuation component is integrated by an integrator 43 to determine an effective current command Irefd.
A voltage amplitude detecting unit 44 takes in the system voltage Vs detected by the voltage detector 29, and outputs a voltage amplitude signal |Vs| showing its voltage value.
A fluctuation detecting unit 45 determines the fluctuation component of the voltage amplitude signal |Vs|, and this fluctuation component is multiplied by a predetermined gain by a proportional computing unit 46 to determine an ineffective current command Irefq.
A PLL (phase-locked loop) circuit 50 is composed of a PLL computing unit 51, an adder 52, and an integrator 53.
This PLL circuit 50 outputs a control reference phase θ. The PLL computing unit 51 receives the zero-crossing signal Z and the control reference phase θ, and outputs a frequency difference Δωs. The adder 52 adds the frequency difference Δωs and a reference angular frequency ωs*, and the resulting sum (Δωs+ωs) is integrated by the integrator 53 to output the control reference phase θ.
With this control reference phase θ as a phase reference, the transforming actions of a dq transformer 60 and a dq inverse transformer 65 to be described later are performed.
The dq transformer 60 carries out dq transformation of the AC output current Iinv detected by the current detector 30 to output the effective component Iinvd of the AC output current and the ineffective component Iinvq of the AC output current.
A subtracter 61 outputs the effective component Δd of a current deviation which is a deviation between the effective current command Irefd and the effective component Iinvd of the AC output current. A current control unit (ACR) 62 performs the PI (proportional plus integral) computation of the effective component Δd of the current deviation to output an effective voltage command Vd.
A subtracter 63 outputs the ineffective component Δq of a current deviation which is a deviation between the ineffective current command Irefq and the ineffective component Iinvq of the AC output current. A current control unit (ACR) 64 performs the PI (proportional plus integral) computation of the ineffective component Δq of the current deviation to output an ineffective voltage command Vq.
The dq inverse transformer 65 carries out the dq inverse transformation of the effective voltage command Vd and the ineffective voltage command Vq to output a voltage command V*.
The PWM (pulse width modulation) modulator 25 PWM-modulates the voltage command V* to produce the gate signal g, and the power converter 26 acts in response to this gate signal g.
As a result, when the frequency signal ωs declines, power compensation is made such that effective power is outputted from the power converter 26, or when the voltage amplitude signal |Vs| declines, power compensation is made such that reactive power is outputted from the power converter 26.
As shown in FIG. 4, the system voltage Vs at the point of connection between the micro grid 10 and the power system 1 is detected to detect the interval between the zero-crossings of the sinusoidal waveform. In the case of this mode, a delay of one period occurs during detection of the frequency. As a result, a control gain cannot be set at a high value. This poses the problem that compensation is insufficient, resulting in a great frequency fluctuation.
As a measure for correcting a deficiency in compensation due to such a delay in the detection period for the frequency, a conventional technology is available which speeds up frequency detection by determining the vector of voltage from the instantaneous value of a three-phase AC voltage, instead of determining the zero-crossings of the voltage waveform, and further utilizing PLL (phase-locked loop) computation.
A self-supporting control unit of a system stabilizing device using such a conventional technology for speeding up frequency detection will be described by reference to FIG. 5.
With a self-supporting control unit 100 shown in FIG. 5, a dq transforming unit 101 dq-transforms a three-phase system voltage Vs to output an effective system voltage Vsd and an ineffective system voltage Vsq of a rotating coordinate system. A polar coordinate trans formation unit 102 performs the polar coordinate transformation of the effective system voltage Vsd and the ineffective system voltage Vsq to output a voltage amplitude signal |Vs| and a phase difference signal φs showing the phase difference of the system voltage Vs with respect to the control reference phase θ.
A fluctuation detecting unit 103 is a filter having differential characteristics and first-order lag characteristics, and outputs the fluctuation component of the voltage amplitude signal |Vs|. This fluctuation component is multiplied by a predetermined gain by a proportional computing unit 104 to determine the ineffective current command Irefq.
A PLL (phase-locked loop) circuit 105 is composed of a proportional plus integral (PI) computing unit 106, an adder 107, and an integrator 108.
The proportional plus integral (PI) computing unit 106 performs the PI (proportional plus integral) computation of the phase difference signal φs of the rotating coordinate system to output a frequency difference Δωs of the rotating coordinate system. The adder 107 adds the frequency deviation Δωs of the rotating coordinate system and a reference angular frequency ωs* of a fixed coordinate system to output an estimated frequency ωs. The integrator 108 integrates the estimated frequency ωs to output the control reference phase θ.
A fluctuation detecting unit 109 is a filter having differential characteristics and first-order lag characteristics, and outputs the fluctuation component of the estimated frequency ωs. This fluctuation component is multiplied by a predetermined gain by a proportional computing unit 110 to determine the effective current command Irefd.